This invention relates to method and apparatus for securing a wheel member of a turbomachine to a rotatable shaft, and in particular to an arrangement which positively secures the wheel to the shaft without generating torsional forces.
Rotors of turbomachinery, such as centrifugal compressors and axial flow compressors and turbines, are at times manufactured as a composite structure, wherein the discs or wheels of the turbomachine are attached to stub shafts, which are fixed to the ends of a thru-bolt, sometimes referred to as a tie-bolt. In achieving the composite structure, heat has been applied to the thru-bolts for developing forces to positively secure the wheels to the shafts. In order to apply the heat, holes have been drilled through the entire length of the shafts, destroying the integrity of the shafts and increasing the problems associated with achieving dynamic and static balance of the rotor. In addition, alignment of the shaft journals has been difficult to achieve without complicated and relatively expensive manufacturing techniques. It has been suggested that the heating step used to positively secure the wheels to the shafts be eliminated, and that compression and torsional forces developed through the use of appropriate bolt and nut arrangements be used to achieve the desired joining of the disc and shaft.
However, turbomachinery employed in many applications, such as power recovery installations, operate at relatively high temperatures. Thus, the working strength of the various elements of the turbomachine are reduced as the working strength of a metallic component generally varies inversely to the temperature of the environment in which the component operates. The torsional forces developed in securing the disc to the shaft will generate torsional stresses. Such stresses may result in the failure of one or more of the bolts due to the relatively low value of the working strength of the bolts. Generally speaking, the shear strength of the bolts to withstand the torsional stresses is approximately one-half the tensile strength of the bolts at any given temperature. As is obvious, a failure of a connecting bolt may result in major damage to the turbomachine.